The present invention relates to particulated solid thermoplastic polymers having applied thereto a uniform coating consisting of zinc stearate. More particularly, the present invention relates to such a composition of matter having improved lubricity properties and demonstrating reduced dusting or generation of fine particulates either of thermoplastic or of zinc stearate.
Zinc stearate is a known lubricant which is added to numerous synthetic thermoplastics often by compounding or incorporation in a uniform homogeneous manner into the polymer. This is often done by melt blending the preformed polymer and zinc stearate or by addition of zinc stearate during polymerization of the thermoplastic polymer. Suitable methods of compounding in the melted state include the use of compounding extruders, ribbon blenders, double-cone blenders and the like. Alternatively, the zinc stearate may be added to the polymer in a nonuniform manner by measuring the desired amount of solid zinc stearate in a powered or particulated form and dispersing the same in the particulated thermoplastic polymer.
Where zinc stearate is added merely by incorporation of a solid powder in the particulated thermoplastic polymer, transport and handling of the product generally results in separation or settling of the particulated lubricant, and variations in the amount of lubricant and polymer thereby result. This may create nonuniform properties in the resulting polymer melt. Clearly, such a result is greatly undesired.
To overcome this defect zinc stearate is often added to the polymer by one of the previously mentioned techniques to obtain homogeneous mixing. However, these previously known methods for incorporating zinc stearate into thermoplastic polymers have associated therewith other undesirable consequences. Regardless of the method for adding the lubricant, normal abrasion forces encountered by the polymer particles during transport and conveying thereof result in the breakage of the particles or granules thereby creating "fines" i.e. micron sized particles of polymer. In other words, the lubricant, even if highly effective after the polymer is melted, is unable to reduce frictional forces during bulk shipping of the solid particulated polymer because the zinc stearate is not concentrated on the surface of such particles. Even if a surface coating of the lubricant is applied in some manner such that the lubricant does not uniformly and completely coat the surface of such particles, the exposed surfaces thereof may remain subject to dust generation due to the foregoing reasons. Accordingly, whereas zinc stearate incorporated uniformly in the particle by the aforesaid melt blending or other technique achieves a product having suitable molding properties under ideal conditions, under actual use conditions, the overall injection molding properties of such product have been found to be adversely affected due to the presence of the aforementioned fines generated during the shipping and transport thereof. Such adverse effects include uneven polymer melting and agglomerate formation in the molten polymer resulting in erratic flow and injection properties and reduced injection molding rates.
In addition, these fines can also enter the atmosphere thereby producing a fire and inhalation hazard. Accordingly, filtration equipment must be installed near particulated polymer conveying equipment to reduce the amount of such particles. Plugging off of such filters and frequent changing thereof due to excessive levels of dust generation are highly undesirable.
It would be desirable to reduce or eliminate such problems caused by generation of fines.
It would further be desirable to provide a thermoplastic resinous product which after transport or conveyance by polymer handling equipment does not contain therein excessive amounts of fines.